It is well known that logging tools and measurement-while-drilling (MWD) tools, which make measurements while traversing deep well boreholes, encounter large variations in borehole temperatures. In general, temperatures increase with depth, and very high temperatures are frequently encountered.
Many types of logging tools and several types of MWD tools contain scintillation detectors for measuring radiation. These include density tools, natural-gamma tools, carbon-oxygen tools, neutron-gamma porosity tools, and certain types of neutron-neutron porosity tools. Scintillation detectors incorporate scintillators for converting gamma rays or charged particles to light, photomultiplier tubes for converting the light to electronic signals, and electronics for processing the electronic signals. Some scintillation detectors can also detect non-ionizing radiation such as neutrons; they incorporate scintillators that are loaded with a special material, the purpose of which is to convert non-ionizing radiation into ionizing radiation that can be detected by the scintillator. Although all of these detector components are subject to variation with temperature, the present art of electronics is such that the electronics can be designed to be relatively insensitive to temperature within the desired operating range. However, the scintillator and photomultiplier tube are not insensitive to temperature variations which cause large fluctuations in detector response. In order to minimize these source fluctuations, it is common to monitor the output of the detectors with electronics and adjust either the high voltage used to operate the detector or adjust the gain of the electronics to account for these variations. (This is called gain stabilization.) However, these corrections do not account for non-linear variations in the detector response that can cause significant errors when large variations in temperature are encountered.
One of the types of logging tools that employs scintillation detectors is designed to measure formation density. Consider a typical density tool in which cesium-137 bombards the formation with gamma radiation. Two scintillation detectors in the tool respond to the gamma rays returned to the tool by the formation. These detectors, with the aid of associated electronics, convert the gamma rays to electronic pulses of varying amplitude, with higher-amplitude pulses corresponding to higher-energy gamma rays. In a typical application, pulses are grouped into windows according to amplitude (and hence energy), and the number of pulses in each energy window for a particular time interval is determined. However, as the temperature of the detector changes, so do the amplitudes of the pulses. In order to keep the pulses correlated to energy, a small cesium-137 stabilization source is usually positioned near the detector, and energy windows are set up to monitor count rates in energy windows in the vicinity of 662 keV, which is the predominant energy of gamma rays that are emitted from the stabilization source. (Gamma rays that originated in the logging source lose enough energy in the formation that they do not make a large contribution to these windows.) These count rates are monitored to determine the pulse amplitude that corresponds to 662 keV gamma rays. If the amplitude deviates from the nominal value, then the high voltage on the detector is adjusted to keep the amplitude at its nominal level. Since this technique only monitors 662 keV gamma rays, voltages corresponding to other energies may drift due to non-linear variations in the detector or electronics.
Although other tools containing scintillation detectors may use other energies and techniques to stabilize the gain or may not stabilize at all, they are all susceptible to non-linear variations with temperature manifest in the drift in amplitude to different windows.
The present apparatus is therefore summarized as a method and apparatus for making corrections to calculations dependent on pulse amplitude and count using measurements from scintillation detectors (and that includes the affiliated photomultiplier tube and appropriate electronics) to account for both linear and nonlinear temperature variations in the response of the detectors.